I am trying to send emails with Office 365, using the method described in "C#/ASP.NET/ASP MVC - Send email using Microsoft OAuth 2.0 (Modern Authentication) + EWS protocol from Office 365 in background service" (https://www.emailarchitect.net/easendmail/ex/c/24.aspx).

I followed the instructions but the connection is always refused by Exchange with a generic "401 Unauthorized" error.

I have queried in Azure web portal the Users > Access Log entries and found the following explanation for the rejected attempts:

"The password entered exceeds the maximum length. Please reach out to your admin to reset the password.
The user is unable to login because their password exceeds the permitted maximum length. They should contact their admin to reset the password. If SSPR is enabled for their tenant, they can reset their password by following the "Forgot your password" link."

The token is really very long and for sure exceeds 256 characters.

Please, can you tell me where I'm wrong?

Thanks a lot!